1. Field of the Invention
The present invention relates generally to improvements in a satellite communications system, and more specifically to a method of generating a ringing signal in a DAMA (demand assigned multiple access) satellite communications system. A communication in the system is established on a SCPC (single channel per single carrier) basis. The present invention is able to provide the system with flexibility in terms of a system design in that a ringing signal at a given terminal station is changed without difficulty.
2. Description of Prior Art
Before turning to the present invention it is deemed preferable to describe, with reference to FIG. 1, a known satellite communications system to which the present invention is applicable.
In FIG. 1, the arrangements which are not directly concerned with the present invention are omitted merely for the sake of simplifying the disclosure.
As shown, a DAMA satellite communications system 10, which establishes a communication on a SCPC basis, includes a satellite 12, a central station 14, and a plurality of terminal stations (only two stations 16a and 16b are shown in this particular case). It is assumed that the terminal stations 16a and 16b are respectively located in different countries.
The central station 14 is provided with a controller 18 and a memory section 20. The controller 18 is arranged to supervise or control overall operations of the system while the memory section 20 stores data indicative of vacant channels which are available in the system 10.
The terminal station 16a is provided with a connection controller 22a and a ringing signal generator 24a. Similarly, the terminal station 16b has a connection controller 22b and a ringing signal generator 24b. Further, a plurality of subscriber terminals (only one is denoted by 26a in the figure) are coupled to the terminal station 16a. Similarly, a plurality of subscriber terminals (only one is denoted by 26b in the figure) are coupled to the terminal station 16b.
It is assumed that the subscriber terminal 26a of the terminal station 16a requests a call to the subscriber terminal 26b of the other station 16b. That is, the terminal stations 16a and 16b are respectively a calling and called station in this case. In order to establish a communication link between the terminal stations 16a and 16b, the station 16a transmits a call request to the central station 14 via a control channel CC1.
The controller 18 of the central station 14, in response to the call request from the station 16a via the satellite 12, determines if a pair of vacant speech channels is available. This check is accomplished by the controller 18 which refers to the memory section 20 which stores vacant channel data therein. It is assumed that a pair of speech channels VC1 and VC2 is available. Thus, the central station 14 advises, via another control channel CC2, the calling and called stations 16a and 16b that the speech channels VC1 and VC2 are to be used for communication between the terminal stations 16a and 16b.
In response to the above-mentioned replay relayed from the central station 14 via the control channel CC2, the connection controllers 22a and 22b initiate to establish the communication link between the stations 16a and 16b. At this time, the connection controller 22b of the terminal station 16b energizes the ringing signal generator 24b. Thus, a subscriber at the terminal 26b is alerted by the ringing signal applied to the subscriber terminal 26b.
A ringing signal is an AC (alternating current) or DC (direct current) signal sent out from the terminal station to a subscriber terminal.
It is known in the art that a ringing voltage, the frequency thereof and the interval (viz., period) of the application of the ringing voltage to the subscriber terminal are different with different countries. Throughout the instant disclosure, the aforesaid electric characteristics of a ringing signal are referred to as a ringing signal pattern.
When the terminal stations 16a and 16b are initially installed, each of the ringing signal generators 24a and 24b is adapted such as to exhibit the ringing signal (viz., ringing signal pattern) which is required in the area or country in which the terminal station is located. Each of the ringing signal generators 24a and 24b is a circuit which is usually prepared using a plurality of discrete electric components in order to generate a required ringing signal.
Therefore, the above-mentioned prior art has encountered the drawback in that the ringing signal generator is fixedly adjusted to produce a predetermined ringing signal and thus is very difficult to be changed to generate a different signal if such a requirement occurs.